Well, it could be that he got it as a gift from some super - advanced ninja technology group in the fanfiction world. Maybe they saw his need for an extra weapon or tool during battles and decided to equip him with a metal arm.
One special ability could be enhanced strength. The metal arm might be able to deliver much more powerful punches than his normal arm, allowing him to break through tough defenses more easily.
For a Naruto character with a metal arm, say it's a new and unknown shinobi. Maybe they were born into a family of blacksmiths who were secretly working on creating advanced weaponry. An accident during the creation of the metal arm led to the young shinobi losing their original arm. But instead of giving up, they decided to use the prototype metal arm. They then trained hard to master its use and became a force to be reckoned with in the Naruto world, using their unique arm to surprise opponents in battles.
One possible character could be Rock Lee. If he had a metal arm, it could enhance his taijutsu. Maybe he got it after a dangerous mission where he lost his original arm. He could use the metal arm to deliver more powerful punches and even incorporate some unique techniques. For example, he could attach weights or blades to it for different combat situations.
Yes, there might be. The Harry Potter fandom is vast and diverse, and fans often come up with creative and unique concepts like this for their fanfictions. There could be a writer who decided to give Harry Potter a metal arm in their story for various reasons, perhaps as a result of a battle injury or some magical - mechanical experiment gone wrong.
In some fanfictions, it could be a gift from a parallel magical dimension. There could be a race in this other dimension that is very advanced in combining magic and metalwork, and they see Harry as a hero worthy of such a gift. The metal arm could then hold the secrets and powers of that dimension, which Harry has to figure out how to use to save the wizarding world.
Naruto may become more cautious. With a prosthetic arm, he realizes his vulnerability more. He might start to think more strategically before jumping into battles. For example, in a fight, he won't just rely on his brute strength but use the unique features of his prosthetic arm to his advantage.
Sasuke could be a candidate. Given their complex relationship, Sasuke might find a way to give Naruto a new arm as a way to atone for their past differences. He could have discovered some rare medical ninjutsu during his travels or come across a powerful artifact that he uses to create a new arm for Naruto.
One possible plot could be that Harry gets his metal arm during a battle with Death Eaters. The arm could be magically enhanced, with runes inscribed on it that give him new abilities like being able to sense dark magic from a distance. He then has to learn to control these new powers while also dealing with the prejudice from some in the wizarding world who see his metal arm as something 'unnatural' in a magical setting.
His metal arm could come from a cursed object that backfired on him. For example, he might have been trying to disarm a powerful dark magic item and it ended up fusing with his arm, turning it into metal. This would then force him to learn how to control the new power within the metal arm.